A mobile device complying with various communication methods switches base stations as a communication target according to the communication method. For example, the mobile device performs voice communication with a base station complying with 3rd Generation (3G) and performs data communication with a base station complying with Long Term Evolution (LTE).
If a voice call is generated during the data communication with an LTE base station, an LTE system performs Inter-Radio Access Technology (RAT) handover with respect to a 3G system. The Inter-RAT handover is handover performed between different communication methods. In the LTE system, control for enabling a voice call in combination with the 3G system is referred to as Circuit Switched (CS) Fallback control.
In a radio communication system in which the CS Fallback control is performed, if the Inter-RAT handover from the LTE system to the 3G system occurs, the base station on the LTE system side transfers data, which is not yet sent to a mobile device, to the base station on the 3G system side. Due to the above-described transfer, the radio communication system may prevent a loss of the data received on the LTE system side by the mobile device before the transfer. The transfer of the unsent data in the Inter-RAT handover is referred to as data forwarding. For example, International Publication Pamphlet No. WO 2008/114449, Japanese National Publication of International Patent Application No. 2010-522466, and Japanese Laid-open Patent Publication No. 2011-15306 disclose techniques related to the data forwarding in the handover between base stations.